The present invention relates to a dry cell which is relatively small in size and has a cylindrical appearance and to a method of manufacturing an outer metal jacket of the dry cell.
Since the material for outer jackets of a small cylindrical dry cell was changed from paper to metal, its ability to prevent leakage has been improved thus increasing the performance of the dry cell. Such a metallic outer jacket of a dry cell (referred to as simply an outer metal jacket) is commonly fabricated in such a producing process as shown in FIG. 6. The procedure starts with cutting a hoop material of about 0.2 mm thickness made of tin-plated steel into rectangular base sheets 1 of predetermined dimensions, for example 785 mm wide and 850 mm long, as shown in FIG. 6(a). This is followed by slitting and trimming two edges of about 2 mm width along both opposing sides of the base sheet 1. After removing the two edge pieces 2, slits are formed which are equally spaced and parallel to the sides of the base sheet 1 as denoted by the two-dot chain lines in FIG. 6(b). The base sheet 1 is then cut along these slits into a group of intermediate strips 3 as shown in FIG. 6(c). Each intermediate strip 3 is punched out with a blanking die as shown in FIG. 6(d), thus producing a number of metal jacket blanks 4, one of which is shown in FIG. 6(e).
The metal jacket blank 4 is then rolled to a cylindrical shape as shown in FIG. 6(f). While two side edges 4a and 4b of the blank 4 are adjoined to each other, a base 4c is inwardly crimped to form a bottom 5, thus making a cylindrical outer metal jacket 7 with a bottom. The outer metal jacket 7 is loaded through the open end thereof with a cell member which comprises a power generating element filled in a zinc can and is coated with an insulating tube. The upper end 4d of the outer metal jacket 7 is then inwardly crimped to be pressed via an insulating ring against the shoulder of the cell member, thus producing a small cylindrical dry cell 8 as shown in FIG. 5.
The rectangular blank 4 punched out from the intermediate strip 3 has notches 9a, 9b at four corners thereof formed by removing triangular segments as shown in FIG. 6(e). When the blank 4 is rolled and its base 4c is crimped inwardly or its upper end 4d is crimped inwardly with the cell member loaded therein, each pair of notches 9a or 9b, at the upper or lower side are adjoined to each other along a joint 10 between the two side edges 4a and 4b or in the radial direction R of the outer metal jacket 7, forming a curled abutment joint 11 as shown in FIG. 5.
More specifically, for forming the curled abutment joint 11, the periphery of the outer metal jacket 7 at its upper end is pressed into a specific semicylindrical curling mold which has a curved cross section. As the upper end of the outer metal jacket 7 is gradually bent to form an even curve all around its periphery, the opposing notches 9aare steadily adjoined to each other from the outer side to the inner side. When the notches 9a are bent to abut each other, a considerable amount of stress is locally exerted along the curled abutment joint 11 and the bending stress is concentrated thereon thus causing buckling.
Further, the cell member loaded in the outer metal jacket 7 varies in height. If the cell member is short, an extra stress is given after the two notches 9a are attached to each other to form the curled abutment joint 11. Such locally concentrated bending stress causes dents 12 on either side of the abutment joint as shown in FIG. 5, which damage the appearance of the outer metal jacket 7. Also, the two notches 9a are fixedly pressed against each other along the curled abutment joint 11, thus firmly pressing a seal member of the cell, which may obstruct the discharge of a gas from the cell in case the gas pressure inside the cell 8 soars unexpectedly.
In order buckling, an improved cylindrical dry cell has been proposed in which four corners of an outer metal jacket blank at upper and lower ends of both sides are rounded off by removing segments of different lengths or heights which are determined in a specific ratio (as disclosed in Japanese Utility-model Publication No. 1-19053). However, it is necessary to cut the blank of the cylindrical dry cell to a desired shape under precise control of the blanking operation. Especially for producing dry cells of small size, it is difficult to precisely control the configuration of tiny blanks when punching them out. On the other hand, the outer metal jacket for dry cells of larger size is jointed by laser welding, whereby the buckling is prevented. However, laser welding is generally not utilized for producing small dry cells because of its low productivity and high producing cost.
In order to form the small notches 9a, 9b having a desired configuration at four corners of the metal jacket blanks 4 in the production process of the outer metal jacket 7 described above, there is virtually no alternative but to punch out a plurality of blanks 4 from the rectangular intermediate strip 3 which is slightly greater in width than a pair of blanks 4 arranged side by side as shown in FIG. 6(d). When the dry cell is large in overall size, the notches may be provided by cutting off four corners of a rectangular strip. However, the notches 9a, 9b of the blank 4 for smaller dry cells are normally minute, i.e., have a height h of 2.2 mm along the sides 4a and 4b and a width d of 0.35 mm from the sides 4a and 4b. The cutting tool of the cutting machine for providing such minute notches 9a, 9b must be extremely sharp and has a short life. Moreover, it is impossible in practice to securely hold a tiny metal jacket blank 4 precisely at a given position.
Accordingly, a waste of material is left in a large amount after the intermediate strip 3 is punched out as can be seen from FIG. 6(d). The number of the blanks 4 punched out from the intermediate strip 3 is limited thus being inefficient in production. In light of the mass production of small dry cells in recent years, such method as described above which creates the loss of material is unfavorable from the economical point of view.
It is thus an object of the present invention to provide a dry cell having such a configuration that any dents caused by the buckling when crimped is effectively prevented, and a method of producing the outer metal jackets efficiently.